It is known to produce three-dimensional objects of complex shape by a process known as stereolithography. Stereolithography refers generally to a process which combines the technologies of optical scanning, laser, polymer chemistry and software. U.S. Pat. No. 4,575,330 to Hull, entitled Apparatus for Production of Three-Dimensional Objects by Stereolithography, describes stereolithography as a method and apparatus for making solid objects by successively printing thin layers of a curable material, such as a UV curable material, on top of each other. According to the above patent, a programmed movable spot beam of UV light shines on a layer of UV curable liquid and forms a solid cross-section of the object at the surface of the liquid. The cured cross-section is then moved away from the liquid surface by the thickness of one layer. The next cross-section is then formed and adhered to the immediately proceeding layer. This process is repeated to form a three-dimensional object. After the object is formed, the object is removed from the SLA (stereolithography apparatus). The object is then cleaned and post cured.
Semi-hollow objects may also be produced with a SLA. For example, QuickCast.TM. by 3D Systems, Inc. is a process used on 3D Systems, Inc. SLA rapid prototyping equipment. QuickCast.TM. as its name implies, was developed to fabricate investment casting patterns. It produces a semi-hollow three dimensional pattern with a honeycomb internal structure. The semi-hollow build style was developed to facilitate drainage of uncured resin from within the internal passages. The internal voids permit the pattern to collapse upon itself as it begins to expand against a mold during investment casting processing.
Post processing of these investment casting patterns involves the formation of multiple drain holes after the pattern is built. The drain holes may also be incorporated during pattern production. The drain holes enable gravity to drain uncured resin from the internal passages.
After gravity draining uncured resin from the pattern, the pattern is removed from the SLA. Several methods may then be employed to rid internal passages of residual, uncured resin. Known methods to remove internal trapped resin include a low speed centrifuging device, nitrogen pressure between 3 psi to 5 psi, and a vacuum. The pattern is then post cured. Post curing typically involves exposure to ultraviolet light.
While the above methods to remove uncured resin from the internal passages of the pattern are useful, a residue of uncured resin within the pattern often results. This residue solidifies during post curing of the object. This uncured resin may result in undesirable object shrinkage or distortion effects during post curing.
In addition, the solidified, uncured resin results in undesirable, additional mass in the object. This additional mass is detrimental to investment casting processing because sufficient voids must exist within the internal passages of the pattern so that it can effectively collapse during processing. The additional mass due to the solidified uncured resin within the internal passages of the pattern fills the voids and hinders the collapsing process.
Accordingly, there is a need for a method of post processing stereolithographically produced semi-hollow objects such that residual, uncured resin is effectively removed from internal passages of the objects.